Two Independent Light Dilution Corrections for the SO2 Camera Retrieve Comparable Emission Rates at Masaya Volcano, Nicaragua

Matthew Varnam,Ryunosuke Kazahaya,Mike Burton,Martha Ibarra,Ben Esse,Giuseppe Salerno

doi:10.3390/rs13050935

Abstract

SO2 cameras are able to measure rapid changes in volcanic emission rate but require accurate calibrations and corrections to convert optical depth images into slant column densities. We conducted a test at Masaya volcano of two SO2 camera calibration approaches, calibration cells and co-located spectrometer, and corrected both calibrations for light dilution, a process caused by light scattering between the plume and camera. We demonstrate an advancement on the image-based correction that allows the retrieval of the scattering efficiency across a 2D area of an SO2 camera image. When appropriately corrected for the dilution, we show that our two calibration approaches produce final calculated emission rates that agree with simultaneously measured traverse flux data and each other but highlight that the observed distribution of gas within the image is different. We demonstrate that traverses and SO2 camera techniques, when used together, generate better plume speed estimates for traverses and improved knowledge of wind direction for the camera, producing more reliable emission rates. We suggest combining traverses and the SO2 camera should be adopted where possible.

Highlights

Gas emissions from volcanoes are a key indicator of volcanic activity, as their release closely links to magma ascent dynamics [1,2,3]
We describe three different methodologies that correct for light dilution and two that calibrate SO2 camera-measured optical depths into SO2 slant column densities (SCDs) images
The synthetic skyline made using projected space-measured radar topography data closely matches the outline of the edifice in the real SO2 camera imagery, confirming that the techniques used to transform the topography into the camera image plane have provided a true match for the camera images

Summary

Introduction

Gas emissions from volcanoes are a key indicator of volcanic activity, as their release closely links to magma ascent dynamics [1,2,3]. Volcanologists commonly measure sulfur dioxide emission rate due to its high plume and low atmospheric concentration, as well as pronounced UV absorption [6]. It can be used as a proxy for total gas emissions through combination with in-plume gas ratio measurements made by instruments such as FTIR, Multi-GAS or alkali filter packs [7,8,9,10]. To obtain a single SO2 emission rate, scientists must measure an SO2 cross-section of the volcanic plume and estimate the plume speed. A spectrometer measures a series of slant column densities (SCDs) through the plume, which combine to yield the cross-section.

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Conclusion